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How do you mean struggle to keep it in place? I had in mind a shim with a lip on so it can’t drop down inside the headtube but sits nicely on the edge of the bearing’s split ring.
I think if I’m not mistaken that the split ring that comes with the headset reduces the ID of the headset from 32.8 to 31.8mm and 1mm isn’t big enough of a gap for a wire. So aligning the slots on the split ring and the reducer shim gives double space
Edit: 0.5mm actually, either side
I think you would have trouble keeping the reducer in place when preloading/compressing the split ring.
And I don't see why you would need to split the reducer?